It is known that these two items of information are critical factors in the safe and effective operation of any aircraft. Prior to each flight, the pilot of a vehicle of this type must therefore make sure that the mass and the balancing of the aircraft do not exceed limits that are predefined by the manufacturer.
A first device is known, that is not on-board the aircraft itself, that serves to weigh an aircraft when empty, i.e. without any loading (passengers, fuel, . . . ), by arranging the aircraft under very precise conditions on a plurality of actuators, themselves disposed on special plates for avoiding introducing interfering forces. The empty mass is then added to the mass of the load in order to estimate the total mass of the aircraft before take-off and the position of its center of gravity.
Since measuring empty mass in this way is very difficult to perform, it is not performed prior to each flight, given the equipment and the time required, which is highly penalizing for that first device, particularly in terms of precision.
It is therefore preferred to use devices on-board the aircraft, to enable an accurate measurement to be obtained of the mass and the position of the center of gravity prior to each flight. In addition, such devices improve flight safety, since the pilot can then verify information coming from external sources that are not necessarily reliable, and relating to the mass of the load and to the position of the center of gravity.
The principle of such devices consists in determining deformation on a portion of each landing gear and in easily deducing therefrom the total mass and the position of the center of gravity of the aircraft.
U.S. Pat. No. 3,426,586 describes a second device of the above type. It consists in inserting a deformable tube inside the axle of each landing gear, the deformable tube being provided with strain gauges. The deformations measured in each tube by the strain gauges enable the total mass and the position of the center of gravity of the aircraft to be deduced.
Although that second device works, it nevertheless presents various drawbacks, in particular associated with strain gauges since they:                are difficult to put into place;        are sensitive to temperature, which requires the use of means specifically for temperature compensation; and        they need to be recalibrated frequently.        
U.S. Pat. No. 4,312,042 discloses a third device using the bend angle of an element in each landing gear as a direct indication of the mass of an aircraft. That document teaches that the third device implements two inclinometers to determine the bending of the element in question.
Nevertheless, experience shows that arranging two inclinometers on each landing gear presents major difficulties that lead to difficulties in applying that third device.